Chimichuri is a marinade or sauce for meat especially beef.
It's commonly used in South America
It's really a pesto if you used basil instead of parsley

Ingredients -

1 bunch flat leaf parsley
8 cloves garlic, minced
3/4 cup extra virgin olive oil
1/4 cup red wine vinegar
1 tablespoon diced red onion
1 teaspoon dried oregano (optional)
1 teaspoon black pepper
1/2 teaspoon salt

1 - Pulse parsley in processor to chop (I just chopped it with a knife)
2 - Add remaining ingredients and blend

I'd suggest using a blender to mix it.
Spoon on meat after it has cooked.